1. Field of the Invention
The present invention relates generally to steam turbines and, more specifically, to an inner cylinder axial positioning system for improving blading sealing.
2. Description of the Related Art
The principle components of a steam turbine include a rotor which has mounted thereon several rows of rotating blades and a stationary cylinder in which the rotor rotates. The stationary cylinder has several rows of stationary blades which extend inwardly toward the rotor, while the rotating blades extend outwardly toward the inner diameter of the cylinder. Seals are provided between the tips of the stationary and rotating turbine blades and corresponding portions of the cylinder and rotor.
Because of differences in thermal expansion, and because of different anchoring points of the rotor and the stationary parts of turbines, the rotor will be displaced axially relative to the cylinder and stationary blades. As a consequence, the number and type of blade seals are affected, resulting in increased leakage. In addition, turbine element length increases because the space between rotating in stationary parts must be increased. This is of concern on retrofit low pressure elements because efficiency can be increased by increasing the number of stages (or blade rows) and the increased blade path span requirements can encroach on the flow area at the inlet zone. This leads to higher inlet velocities and higher flow distribution losses, resulting in higher inlet pressure drop.
Also, because of the aforementioned relative movement, the axial space between the rotor and stationary blades in the wet steam zone of the low pressure elements is reduced in one half of the element and increased in the other half. It has been observed that increased axial spacing between the rotating and stationary blades of a stage reduces moisture erosion by causing break-up of the large moisture droplets streaming off the trailing edges of the stationary blades. Comparison of erosion depth on the three low.TM.pressure elements of a nuclear turbine has revealed a sizeable difference in the amount of erosion in one half of each of the double flow low pressure elements as compared to the other half of the double flow element.
Seals on stationary blades of more recent designs are usually limited to the straight through type as shown in FIG. 1, where all of the seals have the same diameter and the mating surface is cylindrical. The stationary blade seals are generally referred to by the numeral 20 and the rotating blade seals are referred by the numeral 22. For this application, the rotating blade seals 22 are also of the straight through type. In the case of the rotating blade seals 22 of FIG. 1, the number of seals could be increased to reduce leakage by reducing pitch between seals. However, this can increase the leakage because it reduces the dissipation of kinetic energy (called kinetic energy annihilation factor) leaving a seal, thereby increasing leakage. Moreover, the straight through seals do not dissipate all of the kinetic energy even at large pitches while stepped or staggered seals completely annihilate the kinetic energy. The magnitude of this parameter correlates with the ratio of seal clearance to seal pitch.
In the event of a seal rub, both straight through and staggered or stepped seals experience increases in the leakage area, thereby resulting in increased leakage. The clearance to pitch ratio increases, however, and so the seal leakage for straight through designs increases even more. The staggered seals create a convoluted path for the leakage by varying the diameter of the clearance space either by stepping the seal mating surface, as shown in FIGS. 2, 3 and 4, or by entered digiting seals alternately mounted on the rotating and stationary members, as shown in FIG. 5. In this instance, there is complete kinetic energy annihilation. Consequently, there is a lesser increase in leakage in staggered or stepped seals than on the straight through type. As a result, there is less performance degradation with time on units with stepped seals. In FIG. 2, the seal is known as a spring-loaded labyrinth seal, while FIGS. 3 and 4 represent radial seals for reaction blading of a large turbine. The seal shown in FIG. 5 is simply referred to as a double radial labyrinth seal
FIG. 6 is an illustration of a more recent blade path with stepped or staggered seals 22 over the rotating blades and straight through seals 20 under the stationary blades. The stepped seals 22 on the lower sealing diameter of the rotating blades must be positioned far enough away from the step so that they do not contact it when the rotor moves to the right. This reduces the number of step seals that can be utilized on a given sealing surface length. In the design illustrated in FIG. 6, there are two straight through seals at each diameter or sealing land. This is to ensure that at least one seal is always effective at each land as the rotor moves back and forth axially.
A given number of step seals has less leakage than a larger number of straight through seals. However, because of this axial movement, the application of stepped seals and increasing the number of steps to reduce leakage is limited.